Gheebah and Step of Isteghfar
Once a man who claimed to be a friend of our Fourth Holy Imam, Imam Ali Zainul Abideen (A) came to him and told him that so and so had been saying horrible things about him. Imam (A) replied that he had not heard anything and would not have if the man had not told him. Imam (A) then continued that the person who had said the horrible things was as if he had shot an arrow and missed the target but the man who had told him was as if he had picked up the fallen arrow and hit the target (Imam (A)) with it.
Allah does not like the habit of spreading scandal and gossip even if it were true. In the time of Prophet Musa (A), once there was a severe drought. For months, there had been no rain. The people prayed and prayed but no rain came. When Prophet Musa (A) went to mount Sinai to pray, Allah revealed to him that amongst his people there was someone who spreading scandal and because of this person Allah had not let it rain. Prophet Musa (A) asked Allah who this person was so that he could tell him off for what he had done.
Allah replied that did Prophet Musa (A) want Him to do what He had forbidden for others. (i.e. talk about others).
The Holy Prophet [s] said: "Slander acts quicker against the faith of a Muslim believer than leprosy does against his body."
Usul-I-Kafi, vol. 2, p. 257
Imam Amir-ul-Mu'mineen Mi [a] said: "The listener to backbiting is the same as the backbiter."
Ghurar-ul-Hikam, p. 307
Imam Amir-ul-Mu'mineen Ali [a] said: "The most vicious person is he who looks for other people's defects while he overlooks his own faults."
Tara'if-ul-Hikam, p. 176
Imam Kazim [a] said: "Cursed is the one who backbites against one's brother (fellow Muslim)."
Bihar-ul-Anwar, vol. 74, p. 232
The Holy Prophet [s] said: "Man is influenced by the faith of his friends. Therefore, be careful of whom you associate with."
Bihar-ul-Anwar, vol. 74, p. 192
Imam Sadiq [a] said: "My most beloved brother is he who (makes me aware of) my faults."
Bihar-ul-Anwar, vol. 74, p. 282
Imam Sajjad [a] said: "Beware of the companionship of the sinful, and helping of the unjust."
Bihar-ul-Anwar, vol. 78, p.151
Imam Sadiq [a] said: "He who seeks the company of those who insult the lovers of Allah has surely disobeyed Him, the Sublime."
The first step of Isteghfar is to resolve never to commit the sin again. Then you have to, in the case of backbiting, seek out those you may have talked ill of and ask for forgiveness. Thereafter, one has to seek forgiveness from Allah.